Lithium batteries, particularly lithium secondary batteries, have features such as high energy density and long life, and therefore, lithium batteries are widely used as power supplies for electric appliances such as video cameras, portable electronic devices such as laptop computers and mobile telephones, electric tools such as power tools, and the like. Recently, lithium batteries are also applied to large-sized batteries that are mounted in electric vehicles (EV), hybrid electric vehicles (HEV), and the like.
A lithium secondary battery is a secondary battery having a structure in which, at the time of charging, lithium begins to dissolve as ions from the positive electrode and moves to the negative electrode to be stored therein, and at the time of discharging, lithium ions return from the negative electrode to the positive electrode, and it is known that the higher energy density of the lithium secondary battery is attributable to the electric potential of the positive electrode material.
Regarding positive electrode active material for lithium secondary batteries of this kind, there are known lithium transition metal oxides having layered structures, such as LiCoO2, LiNiO2, and LiMnO2; and lithium transition metal oxides having a manganese-based spinel structure (Fd-3 m) (in the present invention, also referred to as “spinel type lithium transition metal oxide” or “LMO”), such as LiMn2O4 and LiNi0.5Mn1.5O4.
Manganese-based spinel type lithium transition metal oxides (LMO) are produced from inexpensive raw materials, are non-toxic and safe, and have properties highly tolerant to overcharging, and therefore, attention has been paid thereto as the next-generation positive electrode active materials for large-sized batteries for electric vehicles (EV), hybrid electric vehicles (HEV), and the like. Also, since spinel type lithium transition metal oxides (LMO) that are capable of three-dimensional insertion and release of Li ions, have superior power output characteristics compared with lithium transition metal oxides having a layered structure, such as LiCoO2, the spinel type lithium transition metal oxides are expected to be useful in applications where excellent power output characteristics are required, such as batteries for EV and batteries for HEV.
In regard to spinel type lithium transition metal oxides (LMO), for example, a lithium manganese composite oxide represented by composition formula: Li1+xMn2−xOu−yFy (wherein 0.02≦x, 0.1≦y≦u, 3≦(2u−y−1−x)/(2−x)≦4, and 3.9≦u≦4.1), having an average particle size in the range of 1 μm to 20 μm, has been heretofore disclosed in Patent Document 1.
Furthermore, Patent Document 2 discloses a Li—Mn-based spinel compound represented by composition formula: Li1+xMn2−x−yMgyO4 (x=0.03 to 0.15 and y=0.005 to 0.05), having a specific surface area of 0.5 m2/g to 0.8 m2/g, and a sodium content of 1000 ppm or less.
Patent Document 3 discloses, as a lithium manganese composite oxide having a sharp particle size distribution and high fluidity, a lithium manganese composite oxide represented by formula: LixMn2−yMeyO4−z (wherein Me represents a metal element or transition metal element having an atomic number of 11 or higher, other than Mn; x has a value of 0<x<2.0; y has a value of 0≦y<0.6; and z has a value of 0≦z<2.0), having an average particle size of 0.1 μm to 50 μm, an n value based on Rosin-Rammler of 3.5 or more, and a BET specific surface area of 0.1 m2/g to 1.5 m2/g, with 60° or less.
Patent Document 4 discloses a lithium manganese composite oxide obtained by heat treating electrolytic manganese dioxide in an oxygen-containing atmosphere at 400° C. to 900° C., subsequently treating the resultant by washing with water, and then calcining the product together with a lithium compound, characterized in that the lithium manganese composite oxide has a sulfur content of 0.32% by weight or less and an average pore diameter of 120 nm or more, and is represented by formula: Li1+xMn2−xO4 (0.032≦x≦0.182).